Explanation of Oxidation Treatment Techniques:
- Chemical oxidation: The film obtained from chemical oxidation treatment is relatively thin, typically ranging from 0.5μm to 4μm. It is soft, less wear-resistant, and has lower corrosion resistance compared to anodized films. It is generally not recommended to be used alone. Due to its good adsorption capability, it is primarily used as a base layer for paint. Chemical oxidation can be classified into alkaline and acidic types based on the nature of the solution. According to the properties of the film, it can be divided into oxide film, phosphate film, chromate film, and phosphate-chromate film (these types of films are conductive).
- Electrochemical oxidation: Electrochemical oxidation (commonly known as anodizing) is a process in which aluminum and aluminum alloys, under specific electrolytes and process conditions, form an oxide film due to the action of an external current. The formation of a layer of oxide film on aluminum and aluminum alloys is referred to as anodizing (this oxide film is non-conductive).
- Hard anodizing: The mechanism of hard oxide film formation is similar to that of conventional sulfuric acid anodizing. However, to obtain a thick and hard film, an electrolyte temperature of around 0 degrees Celsius, high voltage, and high current methods are required. This ensures that the rate of film formation is much higher than the rate of dissolution, resulting in changes in the structure of the film, which is a new characteristic of the growth process of hard oxide films. Hard oxide films also have a dual-layer structure, with the difference being that the thickness of the barrier layer is 10 times greater than that of a regular oxide film, and the same goes for the thickness of the pore walls. This is one of the main reasons for their high hardness. However, the porosity of hard oxide films is 7-8 times less than that of regular oxide films, ranging from only 2% to 6%. The structure of the hard film is disorderly and interferes with each other, resulting in a unique prismatic shape. This leads to high internal stress in the film, which can even cause cracking, as well as the retention of alloying elements and electrolytic decomposition products in the film walls. This results in darker colors and variations in color depending on the composition of the alloy. The alloy composition and impurities have a significant impact on hard anodizing, affecting the uniformity and integrity of the oxide film. Aluminum-copper alloys, aluminum-silicon alloys, and aluminum-manganese alloys present challenges for hard anodizing.
- Structure of aluminum anodized films: The oxide film consists of a barrier layer and a porous layer. The porous layer is composed of many hexagonal prism-shaped oxide cells (pore cells), with a small hexagonal pore in the center of each cell. This gives it a honeycomb-like structure. The thickness of the pore walls is twice the diameter of the pores. The average porosity of sulfuric acid anodized films is 20% to 30%, with approximately 800 small pores on a surface of 1μm2. This allows for various colors to be achieved after anodizing.
- Coloring of anodized films: Anodized films have a porous structure and chemical activity, making them easily amenable to coloring treatments. Aluminum oxide films have 20% to 30% porosity (in the case of sulfuric acid films), resulting in a large surface area and excellent chemical adsorption capability. Dye molecules are adsorbed and retained in the inner layer of the oxide film through physical and chemical adsorption, resulting in coloration. The lower porosity of hard oxide films makes them more challenging to color, and since they are thicker films, they tend to have a darker base color. Therefore, black coloring is more ideal for hard anodized films.
- Post-sealing treatment of anodized films: Aluminum anodized films have high porosity and adsorption capacity, making them susceptible to contamination, corrosion, and medium erosion. Therefore, regardless of whether the oxide film is colored or not, it must undergo a sealing treatment to improve corrosion resistance, enhance resistance to contamination, and fix the pigments. However, if there are specific subsequent treatments, sealing may not be necessary and the adsorption capacity can be increased.
Considerations for Producing Hard Anodized Films:
- All sharp corners on the workpiece should be rounded to a radius of not less than 0.5mm to avoid local overheating, embrittlement, and fractures caused by concentrated current.
- Areas that do not require a thick film should be protected using materials such as vinyl chloride adhesive. Threaded holes and dowel pin holes should be plugged with plastic or rubber.
- After the workpiece undergoes hard anodizing, its dimensions will increase by approximately half of the film thickness (on one side). Therefore, for products with strict dimensional requirements, the allowable tolerance before anodizing should be determined based on the film thickness.
- The oxide film is firmly bonded to the substrate, but it is brittle. As the thickness increases and the pore size grows, it is not suitable for parts subjected to impact, bending, or deformation. A hard oxide film of a certain thickness can significantly reduce the fatigue strength of aluminum alloys, especially high-strength aluminum alloys. Therefore, hard anodizing should be approached with caution for parts subjected to fatigue loads.